174 research outputs found
An Upper Limit on the Mass of the Circumplanetary Disk for DH Tau b
IndexaciĂłn: Scopus.DH Tau is a young (sim;1 Myr) classical T Tauri star. It is one of the few young PMS stars known to be associated with a planetary mass companion, DH Tau b, orbiting at large separation and detected by direct imaging. DH Tau b is thought to be accreting based on copious Ha emission and exhibits variable Paschen Beta emission. NOEMA observations at 230 GHz allow us to place constraints on the disk dust mass for both DH Tau b and the primary in a regime where the disks will appear optically thin. We estimate a disk dust mass for the primary, DH Tau A of 17.2 ± 1.7 MĂ
, which gives a disk to star mass ratio of 0.014 (assuming the usual gas to dust mass ratio of 100 in the disk). We find a conservative disk dust mass upper limit of 0.42Mâ for DH Tau b, assuming that the disk temperature is dominated by irradiation from DH Tau b itself. Given the environment of the circumplanetary disk, variable illumination from the primary or the equilibrium temperature of the surrounding cloud would lead to even lower disk mass estimates. A MCFOST radiative transfer model, including heating of the circumplanetary disk by DH Tau b and DH Tau A, suggests that a mass-averaged disk temperature of 22 K is more realistic, resulting in a dust disk mass upper limit of 0.09Mâ for DH Tau b. We place DH Tau b in context with similar objects and discuss the consequences for planet formation models.http://iopscience.iop.org/article/10.3847/1538-3881/aa74cd/met
Most ELÂ CVn systems are inner binaries of hierarchical triples
In spite of their importance for modern astronomy, we do not fully understand how close binary stars containing at least one white dwarf form from main sequence binary stars. The discovery of EL CVn binaries, close pre-white dwarfs with A/F main sequence star companions, offers now the unique possibility to test models of close compact binary star formation. Binary evolution theories predict that these EL CVn stars descend from very close main sequence binaries with orbital periods shorter than 3 days. If this is correct, nearly all EL CVn stars should be inner binaries of hierarchical triples because more than 95 per cent of very close main sequence binaries (the alleged progenitor systems) are found to be hierarchical triples. We here present SPHERE/IRDIS observations of five EL CVn binaries, finding in all of them tertiary objects, as predicted. We conclude that EL CVn systems are inner binaries of hierarchical triples and indeed descend from very close main sequence binaries that experience stable mass transfer
Dimensionality dependence of the wave function statistics at the Anderson transition
The statistics of critical wave functions at the Anderson transition in three
and four dimensions are studied numerically. The distribution of the inverse
participation ratios (IPR) is shown to acquire a scale-invariant form in
the limit of large system size. Multifractality spectra governing the scaling
of the ensemble-averaged IPRs are determined. Conjectures concerning the IPR
statistics and the multifractality at the Anderson transition in a high spatial
dimensionality are formulated.Comment: 4 pages, 4 figure
Orbital periods and component masses of three double white dwarfs
The merger of close double white dwarfs (CDWDs) is one of the favourite evolutionary
channels for producing Type Ia supernovae (SN Ia). Unfortunately, current theories of
the evolution and formation of CDWDs are still poorly constrained and have several
serious uncertainties, which affect the predicted SN Ia rates. Moreover, current observational
constraints on this evolutionary pathway for SN Ia mainly rely on only 18
double-lined and/or eclipsing CDWDs with measured orbital and stellar parameters
for both white dwarfs. In this paper we present the orbital periods and the individual
masses of three new double-lined CDWDs, derived using a new method. This
method employs mass ratios, the Hα core ratios and spectral model-fitting to constrain
the masses of the components of the pair. The three CDWDs are WD0028â474
(Porb=9.350 ± 0.007 hours, M1 = 0.60 ± 0.06 Mâ, M2 = 0.45 ± 0.04 Mâ), HE0410â
1137 (Porb= 12.208 ± 0.008 hours, M1 = 0.51 ± 0.04Mâ, M2 = 0.39 ± 0.03 Mâ)
and SDSSJ031813.25â010711.7 (Porb= 45.908 ± 0.006 hours, among the longest period
systems, M1 = 0.40 ± 0.05Mâ, M2 = 0.49 ± 0.05Mâ). While the three systems
studied here will merge in timescales longer than the Hubble time and are expected
to become single massive (& 0.9 Mâ) white dwarfs rather than exploding as SN Ia,
increasing the small sample of CDWDs with determined stellar parameters is crucial
for a better overall understanding of their evolution
Magnetic dynamos in white dwarfs â II. Relating magnetism and pollution
We investigate whether the recently suggested rotation and crystallization driven dynamo can explain the apparent increase of magnetism in old metal polluted white dwarfs. We find that the effective temperature distribution of polluted magnetic white dwarfs is in agreement with most/all of them having a crystallizing core and increased rotational velocities are expected due to accretion of planetary material which is evidenced by the metal absorption lines. We conclude that a rotation and crystallization driven dynamo offers not only an explanation for the different occurrence rates of strongly magnetic white dwarfs in close binaries, but also for the high incidence of weaker magnetic fields in old metal polluted white dwarfs
Accretion signatures in the X-shooter spectrum of the substellar companion to SR12
About a dozen substellar companions orbiting young stellar objects or pre-main sequence stars at several hundred au have been identified in the last decade. These objects are interesting both due to the uncertainties surrounding their formation, and because their large separation from the host star offers the potential to study the atmospheres of young giant planets and brown dwarfs. Here, we present X-shooter spectroscopy of SRâ12âC, a âŒ2âMyr young brown dwarf orbiting SRâ12 at an orbital separation of 1083âau. We determine the spectral type, gravity, and effective temperature via comparison with models and observational templates of young brown dwarfs. In addition, we detect and characterize accretion using several accretion tracers. We find SRâ12âC to be a brown dwarf of spectral type L0 ± 1, logâg = 4 ± 0.5, an effective temperature of 2600 ± 100âK. Our spectra provide clear evidence for accretion at a rate of âŒ10â10âMââyrâ1. This makes SRâ12 one of the few sub-stellar companions with a reliable estimate for its accretion rate. A comparison of the ages and accretion rates of sub-stellar companions with young isolated brown dwarfs does not reveal any significant differences. If further accretion rate measurements of a large number of substellar companions can confirm this trend, this would hint towards a similar formation mechanism for substellar companions at large separations and isolated brown dwarfs
The SDSS spectroscopic catalogue of white dwarf-main-sequence binaries: new identifications from DRÂ 9â12
We present an updated version of the spectroscopic catalogue of white dwarf-main-sequence (WDMS) binaries from the Sloan Digital Sky Survey (SDSS). We identify 938 WDMS binaries within the data releases (DR) 9â12 of SDSS plus 40 objects from DR 1â8 that we missed in our previous works, 646 of which are new. The total number of spectroscopic SDSS WDMS binaries increases to 3294. This is by far the largest and most homogeneous sample of compact binaries currently available. We use a decomposition/fitting routine to derive the stellar parameters of all systems identified here (white dwarf effective temperatures, surface gravities and masses, and secondary star spectral types). The analysis of the corresponding stellar parameter distributions shows that the SDSS WDMS binary population is seriously affected by selection effects. We also measure the NaâI λλ 8183.27, 8194.81 absorption doublet and H α emission radial velocities (RV) from all SDSS WDMS binary spectra identified in this work. 98 objects are found to display RV variations, 62 of which are new. The RV data are sufficient enough to estimate the orbital periods of three close binaries
The nature of transition circumstellar disks. I. The ophiuchus molecular cloud
We have obtained millimeter-wavelength photometry, high-resolution optical spectroscopy, and adaptive optics near-infrared imaging for a sample of 26 Spitzer-selected transition circumstellar disks. All of our targets are located in the Ophiuchus molecular cloud (d ⌠125pc) and have spectral energy distributions (SEDs) suggesting the presence of inner opacity holes. We use these ground-based data to estimate the disk mass, multiplicity, and accretion rate for each object in our sample in order to investigate the mechanisms potentially responsible for their inner holes. We find that transition disks are a heterogeneous group of objects, with disk masses ranging from JUP and accretion rates ranging from JUP) and negligible accretion (<10-11 M âyr-1), and are thus consistent with photoevaporating (or photoevaporated) disks. Four of these nine non-accreting objects have fractional disk luminosities <10-3 and could already be in a debris disk stage. Seventeen of our transition disks are accreting. Thirteen of these accreting objects are consistent with grain growth. The remaining four accreting objects have SEDs suggesting the presence of sharp inner holes, and thus are excellent candidates for harboring giant planets.Facultad de Ciencias AstronĂłmicas y GeofĂsica
Magnetic dynamos in white dwarfs â III: explaining the occurrence of strong magnetic fields in close double white dwarfs
The origin of strong (â âŒ>1MGâ ) magnetic fields in white dwarfs has been a puzzle for decades. Recently, a dynamo mechanism operating in rapidly rotating and crystallizing white dwarfs has been suggested to explain the occurrence rates of strong magnetic fields in white dwarfs with close low-mass main-sequence star companions. Here, we investigate whether the same mechanism may produce strong magnetic fields in close double white dwarfs. The only known strongly magnetic white dwarf that is part of a close double white dwarf system, the magnetic component of NLTTâ12758, is rapidly rotating and likely crystallizing and therefore the proposed dynamo mechanism represents an excellent scenario for the origin of its magnetic field. Presenting a revised formation scenario for NLTTâ12758, we find a natural explanation for the rapid rotation of the magnetic component. We furthermore show that it is not surprising that strong magnetic fields have not been detected in all other known double white dwarfs. We therefore conclude that the incidence of magnetic fields in close double white dwarfs supports the idea that a rotation- and crystallization-driven dynamo plays a major role in the generation of strong magnetic fields in white dwarfs
Magnetic white dwarfs in post-common-envelope binaries
Magnitude-limited samples have shown that 20-25 per cent of cataclysmic variables contain white dwarfs with magnetic fields of Mega Gauss strength, in stark contrast to the approximately 5 per cent of single white dwarfs with similar magnetic field strengths. Moreover, the lack of identifiable progenitor systems for magnetic cataclysmic variables leads to considerable challenges when trying to understand how these systems form and evolve. Here we present a sample of six magnetic white dwarfs in detached binaries with low-mass stellar companions where we have constrained the stellar and binary parameters including, for the first time, reliable mass estimates for these magnetic white dwarfs. We find that they are systematically more massive than non-magnetic white dwarfs in detached binaries. These magnetic white dwarfs generally have cooling ages of more than 1 Gyr and reside in systems that are very close to Roche-lobe filling. Our findings are more consistent with these systems being temporarily detached cataclysmic variables, rather than pre-cataclysmic binaries, but we cannot rule out the latter possibility. We find that these systems can display unusual asymmetric light curves that may offer a way to identify them in larger numbers in future. Seven new candidate magnetic white dwarf systems are also presented, three of which have asymmetric light curves. Finally, we note that several newly identified magnetic systems have archival spectra where there is no clear evidence of magnetism, meaning that these binaries have been previously missed. Nevertheless, there remains a clear lack of younger detached magnetic white dwarf systems
- âŠ